1. Field of Invention
The present invention relates to a brightness adjusting method, and more particularly to a fixed frequency dimming method and a fixed frequency dimming circuit for a light emitting module.
2. Related Art
In recent years, the manufacturing technology of a light emitting diode (LED), one of semiconductor light emitting components, develops rapidly. In particular, since a blue LED is successfully developed, such that LEDs having three primary colors of light are all available, light of various wavelengths may be realized by combining the LEDs of three primary colors. Therefore, the application scope of the LEDs rapidly expands.
As for operational characteristics, brightness of an LED changes with a current. When a high current flows through a light emitting component, a high-brightness light emitting effect is achieved. On the contrary, if the flowing current is reduced, the brightness of the light emitting component is relatively lowered.
In the prior art, an amplitude of a driving current of an LED can be modulated through hysteresis control, thereby changing the brightness of the LED. In other words, a hysteresis DC/DC converter is used to control a level of the driving current of the LED.
A relation formula of the hysteresis control is as shown in Formula One below.
                              F          s                =                                            (                                                V                  in                                -                                  V                  out                                            )                        ×            D                                              Hys              Amp                        ×            L            ×                          I              set                                                          Formula        ⁢                                  ⁢        One            
Fs represents an oscillation frequency, Vin represents an input voltage, Vout represents an output voltage, D represents an input voltage/output voltage, Iset represents an output current (that is, a driving current of an LED), L represents an inductance, and HysAmp represents a hysteresis oscillation width.
During the process for implementing level modulation of an output current, as the input voltage, the output voltage, the inductance, and the hysteresis oscillation width are all maintained at fixed values, the oscillation frequency of the circuit changes with the level of the output current. In other words, decrease of the output current causes proportional increase of the oscillation frequency.
However, when the oscillation frequency changes with the level of the output current, the protection design of electromagnetic interference (EMI) is relatively difficult. Moreover, conversion loss often exists during the DC/DC conversion. When the oscillation frequency increases, the conversion loss increases, resulting in lower conversion efficiency.